1. Field of the Invention
This invention relates to structurally unique polymeric composites, including composites with novel reinforcement systems. This invention further relates to new types of polymeric composites, compomers, resin-modified glass ionomers and glass ionomers that contain novel reinforcement systems as well as ion-releasing fluorosilicate glasses for fluoride release. More particularly, in its preferred embodiment, the invention relates to polymeric dental composites, compomers, resin-modified glass ionomers and glass ionomers containing elongated whisker fillers, including single-crystalline ceramic whiskers, polycrystalline chopped fibers, chopped glass fibers, and chopped polymer fibers. Further, this invention relates to methods and compositions for: (1) blending whiskers and/or chopped fibers with ground pre-cured glass ionomer cements in powder form, and/or ion-releasing fluorosilicate glass and polyacid for fluoride release; (2) mixing whiskers and/or chopped fibers with silicate particulate fillers for improved filler distribution in matrix, thereby minimizing whisker and/or chopped fiber entanglement; and (3) coating and bonding silicate filler particles and/or ion-releasing fluorosilicate glass particles onto the surfaces of individual whiskers or chopped fibers for ease of silanization and improved whisker retention in matrix by providing rougher particle-bonded whisker surfaces. The reinforced material systems are useful for the preparation of dental and medical adhesives, bases, liners, luting agents, sealants, core buildup and direct filling materials, inlays and onlays for restorative uses, as well as for endodontic and orthopedic use.
2. Description of the Prior Art
The dental composite resins can be viewed as one extreme of a continuous spectrum of multi-phase dental materials with the glass ionomers at the other extreme, and hybrid materials such as resin-modified glass ionomers and compomers intermediary in the spectrum. Whereas both composite resins and glass ionomers have been known and used for over two decades, the hybrid materials such as the resin-modified glass ionomers and compomers are relatively new and have only been available for about half a decade.
2.a Polymeric Composites
In the area of dental restorative materials, ceramic filler reinforced polymeric composites are widely used. A typical dental composite is composed of a mixture of silicate glass or quartz particles with an acrylic monomer that is polymerized to form a hardened composite material. In current dental composites, the fillers are mostly glasses, occasionally glass-ceramics and quartz (a crystalline form of silica), particulate polymers and glass-polymer composite particulates. Current strategies for their improvement include the development of smaller filler particles such as microfiller and nanofiller particles, the improvement of glass compositions, and the increase of filler volume fraction through the use of hybrid and heterogeneous filler systems.
Due to concerns about the release of mercury from dental amalgam, there is an increasing need to extend the use of polymeric composites to stress bearing posterior applications. However, the relatively low toughness, strength, wear resistance and durability of dental composites have limited their use. It is generally agreed that polymeric composites "cannot be routinely substituted for dental amalgam and achieve the same clinical results. In the posterior dentition, in situations where occlusal stresses are concentrated, the current composites . . . are inappropriate choices" (Corbin and Kohn, 1994, JADA 125: 381-388). The current polymeric composites "are not recommended for large posterior restorations because of the potential for excessive wear, microleakage or fracture" (Bayne et al. 1994, JADA 125: 687-701). Composite restorations, in low stress-bearing applications not involving cusps, have average lifetimes of less than 10 years. In comparison, dental amalgam restorations, in high stress-bearing posterior applications with cusp replacement, have lifetimes of 15 years (Corbin and Kohn, 1994, JADA 125: 381-388).
2.b Glass Ionomers, Resin-modified Glass Ionomers and Compomers
Radical changes have occurred during the past decade in dental restorative materials. However, none has had a greater impact than the fluoride-releasing glass ionomer materials. Glass ionomer materials are based on the acid-base reaction of an aqueous solution of a polycarboxylic acid with an ion leachable, fluoride-containing glass (Wilson and Kent, J Appl Chem Biotechnol 21: 313, 1971; Wilson and Kent, Br Dent J 132: 133, 1972; Wilson and Kent, Br Pat 1,316,129, 1973; Wilson and Crisp, Br Pat 1,422,337, 1976). Glass ionomers are noted for their inherent adhesiveness to teeth and their ability to release fluoride to adjacent tooth structure in a sustained fashion to combat secondary caries. However, the inferior mechanical properties of glass ionomers, especially their extreme brittleness and low strength (e.g., flexural strength of 10-20 MPa, McLean, J Am Dent Assoc 120: 43, 1990) have severely limited their use. The reinforcement of glass ionomers by disperse phase Corundum (Prosser et al., J Dent Res 65: 146, 1986), alumina fibers and other fibers (Sced and Wilson, Br Pat Appli GB 2,028,855A, 1978), and metal powders (McLean and Gasser, U.S. Pat. No. 4,527,979, 1985) resulted in only incremental improvement in mechanical properties. Flexural strength values of reinforced glass ionomers have rarely exceeded 50 MPa (Wilson and McLean, Glass-Ionomer Cement, 1988; McLean, J Am Dent Assoc 120: 43, 1990).
Resin-modified glass ionomers (Mitra, Eu Pat Appl 323120, 1988; U.S. Pat. No. 5,154,762, 1992), where compatible resins (e.g., 2-hydroxyethyl methacrylate, or HEMA) are used with the polyacids, are only slightly stronger than glass ionomers (e.g., flexural strength of 60 MPa, Poolthong et al., Dent Mater J 13: 220, 1994; Hickel, Acad Dent Mater Trans 9: 105, 1996). It is widely recognized that "the most intractable problem is likely to be lack of strength and toughness" (Wilson and McLean, Glass-Ionomer Cement, 1988).
Recently, a possible breakthrough occurred with the development of compomers (Blackwell et al., U.S. Pat. No. 4,816,495, 1989; Huang et al., U.S. Pat. No. 5,367,002, 1994; Peters et al., J Dent Res 73, 1994; Peters et al., J Dent Res 74, 1995; Barnes et al., J Dent Res 75: 293, 1996; Hickel, Acad Dent Mater Trans 9: 105, 1996; Blackwell et al., Acad Dent Mater Trans 9: 77, 1996). Compomers are basically hybrid, glass ionomer-composites modified in their resin phase by a carboxylic acid monomer and in their filler phase by the inclusion of an acid-reactive, ion-leachable glass. The name compomer is derived by combining the two words composite and ionomer, and is intended to suggest a combination of composite and glass-ionomer technology. The liquid part of a compomer is a mixture of a dental resin monomer (such as UDMA, a urethane dimethacrylate) and a carboxylic acid monomer (e.g., TCB, the reaction product of butane tetracarboxylic acid with HEMA), with the resin being the major phase and TCB the minor phase. The filler part of a compomer is a mixture of dental silicate glass and reactive fluorosilicate glass particles, with the reactive glass being the minor phase. In contrast to glass ionomers, compomers do not contain significant amounts of water. The sole initial curing reaction is radical induced polymerization of the acrylic resin monomer matrix. An acid-base reaction takes place between TCB and the ion leachable fluorosilicate glass only after water infuses the cured composite via exposure to oral fluids, which also causes the filling to release fluoride ions. Flexural strength values of 90-125 MPa have been reported for compomers (Hickel, Acad Dent Mater Trans 9: 105, 1996). However, these strength values are still inferior to those of current dental amalgam (110-150 MPa) and composite resins (100-145 MPa) (Hickel, Acad Dent Mater Trans 9: 105, 1996). Therefore, compomers are currently not recommended for use in large, stress-bearing posterior applications.
2.c Problems in Current Materials
Two major problems have been overlooked in the current research and development of dental composite resins, glass ionomers, resin-modified glass ionomers and compomers. The first problem is that, glasses and glass-ceramics are among the weakest and most brittle materials to use as reinforcement fillers. Glass filler particles are sensitive to surface flaws produced during mixing, handling and wear. A crack in the resin matrix can easily cut through the reinforcing glass particles (lower arrow in FIG. 1). The second problem is related to the geometrical shapes of the filler particles. The current glass fillers are either spherical or of irregular shapes, with length-to-diameter ratio only slightly larger than one. This has at least two major short-comings. First, rounded filler particles at occlusal surfaces are susceptible to facile dislodgement from the resin matrix during wear with foods bolus, resulting in high wear rates. Second, if a crack is initiated in the composite, it can easily propagate around the filler particles (upper arrow in FIG. 1), hence causing the reinforcing effect of the filler particles to be lost.
2.d This Invention: Whisker Reinforcement
There is a need to overcome the problems described above. The present invention uses ceramic whisker reinforcement to improve the mechanical properties of dental composite resins, glass ionomers, resin-modified glass ionomers, and compomers. The term "whisker" is used to include the following fillers of elongated shapes: (1) ceramic single-crystalline whiskers, such as silicon nitride, silicon carbide, mullite, zirconia, sapphire; (2) chopped fibers, including ceramic such as silicon carbide, silicon nitride, alumina, zirconia, carbon, glass fibers, and polymer fibers. The diameter of the whiskers and chopped fibers ranges from 0.1 .mu.m to 300 .mu.m, preferably from 0.2 .mu.m to 20 .mu.m. The length of the whiskers and chopped fibers ranges from 1 .mu.m to 10 mm, preferably from 5 .mu.m to 1 mm, most preferably from 5 .mu.m to 50 .mu.m.
This invention further uses silicate filler particles to bond onto the surfaces of individual whiskers or chopped fibers to: (1) improve the efficacy of whisker and chopped fiber silanization and bonding with the matrix; (2) separate the whiskers and chopped fibers from each other, thereby preventing entanglement; and (3) enhance the retention of the whiskers and chopped fibers in matrix by providing rougher particle-bonded surfaces of the whiskers and chopped fibers. Any glassy or crystalline silicate-containing particles can be used, including dental microfill glass, hybrid glass, quartz, silicon nitride, silicon carbide, and glass ceramics, with particle diameter ranging from 0.01 .mu.m to 100 .mu.m, preferably from 0.03 .mu.m to 10 .mu.m.
In the past, single-crystalline whiskers have been used to reinforce ceramics (e.g., Becher and Wei 1984, J Am Ceram Soc 12: C267-269; Hirata et al., 1989, J Ceram Soc Jpn 97: 866-871) and metals, Bose et al. 1994, U.S. Pat. No. 5,116,004). Certain plastics (or resins) have also been reinforced with ceramic whiskers. Whisker-reinforced plastics have been proposed for use in applications including electrical (Robeson and Harris, 1986, U.S. Pat. No. 4,613,645), golf club head (Tominaga and Sasaki, 1987, U.S. Pat. No. 4,687,205), and orthodontic bracket (Carberry and Negrych, 1992, U.S. Pat. No. 5,078,596). However, single-crystalline whiskers have not been used to reinforce dental direct filling or indirect polymeric composites, glass ionomers, resin-modified glass ionomers or compomers.
Further, in the past, chopped glass fibers and crystalline fibers and polymer fibers have been used to reinforce certain polymers (Grant and Greener, Aust Dent J 12: 29, 1967; Skirvin et al., Military Med 147: 1037, 1982; Krause et al., J Biomed Mat Res 23: 1195-1211, 1989; Williams et al, 1992, Dent Mater 8: 310-319; Bayne and Thompson, 1996 Academy Dent Mater 9: 238). However, silicate particle fillers have not been used to mix with chopped fibers to disperse the fibers preventing entanglement, and the individual chopped fibers have not been bonded or coated with silicate filler particles to improve filler distribution and enhanced retention in the matrix. This is likely why dental composites reinforced with chopped fibers have not showed significant improvements over silicate particle reinforced composites (Williams et al, 1992, Dent Mater 8: 310-319; Bayne and Thompson, 1996 Academy Dent Mater 9: 238).
To conclude, in the past, single-crystalline ceramic whisker and polycrystalline chopped fibers and glass chopped fibers and polymer fibers have not been individually bonded or coated with silicate filler particles and/or ion-releasing fluorosilicate glasses to reinforce dental polymeric composites, glass ionomers, resin-modified glass ionomers and compomers. Further, single-crystalline ceramic whisker and polycrystalline chopped fibers and glass chopped fibers and polymer fibers have not been mixed with ion-releasing fluorosilicate glasses and/or polyacid and/or powdered pre-cured glass ionomers and/or powdered pre-cured resin-modified glass ionomers to reinforce dental materials.